1. Field of the Invention
This invention relates to a valve control system to position a valve in response to an electrical command signal.
2. State of the Art
Valve control systems typically position a valve in response to a command signal, generally an electrical signal generated outside the valve control system. The current level of the electrical signal represents the desired position of the valve. For example, many valve control systems respond to command signals that vary between 4 to 20 milliamperes, with 4 milliamperes representing the valve in the fully closed position, 20 milliamperes representing the valve in the fully opened position, and a signal between 4 and 20 milliampere linearly representing a partially opened (or closed) valve.
Many valve control systems also include a valve sensor for detecting the position of the valve and generating an electrical feedback signal representing the present position of the valve. A command circuit compares the electrical feedback signal to the command signal and generates a correction signal. The correction signal, after processing, directs an electro-pneumatic device, such as the block and spool described in the detailed description below, to direct pressure to a pneumatic actuator that physically moves the valve to the desired position.
Present valve control systems have a lag time between the receipt of the command signal and the actual placement of the valve. The present systems also have an inability to precisely hold a valve in a partially opened position in response to a proper command signal. There is a need for an improved valve control system with a quicker response time and improved position control.